1. Field of the Invention
The present invention relates to a thermal head and, more particularly, to an improvement in a protective layer of the thermal head.
2. Description of the Related Art
In recent years, a thermal head has been widely used in various recording devices such as a facsimile device and a word processor printer since the thermal head has advantages of noiseless, no need of maintenance, and a low running cost. Since such a recording device is required to be more compact, less expensive, and consume less power, a compact, inexpensive, and high-performance thermal head is also desired.
In order to satisfy the above requirements, Japanese Patent Disclosure (KOKAI) No. 52-100245 discloses a method in which a resin having a small thermal conductivity such as a polyimide resin or an epoxy resin is used as a heat insulating layer instead of conventional glazed glass. Since these resins have a low thermal diffusivity, the thermal heads using the resins have a high efficiency and can be easily bent to realize a compact size. A thermal head using such a polyimide resin as a heat insulating layer, however, cannot perform a stable printing operation for a long time period. The reasons for this are as follows. First, no polyimide resin having a sufficient heat resistance against an operation temperature of a thermal head can be obtained. Second, no sufficient adhesion between a resin and a substrate and between the resin and a thin film formed on the resin can be obtained not only under the static condition but also under the one of the repeated thermal stress.
The present inventors, however, have recently developed a siloxane-modified aromatic polyimide resin having a molecular structure represented by formula (1) as a material of a heat insulating layer so that a thermal head using a resin as a heat insulating layer can be put into practical use. ##STR1##
A detailed structure of a thermal head of this type will be described below with reference to FIG. 1. Referring to FIG. 1, reference numeral 1 denotes a metal substrate consisting of, e.g., an Fe-Cr alloy; and 2, a layer consisting of a polyimide resin represented by (formula 1). The polyimide resin layer 2 is obtained by coating and baking polyamic acid on the metal substrate 1. Polyamic acid is synthesized by substituting 0.05 to 10 mol % of p-phenylene diamine by bisaminosiloxane upon ring-opening poly-addition reaction of an equimolar mixture of a biphenyl tetracarboxylic acid dihydride and p-phenylene diamine. Reference numeral 3 denotes an undercoating layer consisting of, e.g., SiO.sub.x, SiN, or SiC. The undercoating layer 3 is formed in order to protect the polyimide resin layer against chemical dry etching or ashing, facilitate control of a resistance upon formation of a heat-generating resistive layer 4, and improve a wire bonding property. Reference numeral 4 denotes a heat-generating resistor consisting of, e.g., Ta-SiO.sub.2 or Ti-SiO.sub.2. Discrete electrodes 6 and a common electrode 7 consisting of, e.g., Al or Al-Si-Cu are formed on the heat-generating resistor 4 so as to form an opening to serve as a heat-generating portion 5. A protective layer 8 consisting of, e.g., Si--O--N, SiN, or SiC is formed so as to cover at least the heat-generating portion 5. The protective layer 8 is illustrated as a single layer in FIG. 1. In an actual structure, however, a plurality of layers, such as an oxidation-proofing layer and an abrasion-proofing layer may be formed independently from each other, or an oxidation-proofing/abrasion proofing layer and an adhesive layer may be formed.
It is confirmed that such a thermal head can sufficiently withstand an operation as a thermal head in terms of a heat-resistance and an adhesive force. When this thermal head is incorporated in a device such as a facsimile device to perform a running test, however, a resistance abnormally changes to adversely affect a printing performance during the test. A singular point of a function at which the resistance abnormally changes as described above was carefully checked. As a result, it is found that hard foreign matters such as dust caught between the thermal head and heat-sensitive paper often causes a crack in the protective film, and the singular point of a function is produced when the crack reaches the heat-generating resistor. In addition, it is found that when a conventional high-resistance substrate obtained by forming glazed glass on Al.sub.2 O.sub.3 or a high-resistance substrate obtained by forming a glass layer on a metal substrate is used, the above phenomenon does not occur even if the other arrangements are the same. That is, it is found that this phenomenon particularly occurs when a resin is used as the heat insulating layer. It is assumed that the phenomenon occurs because a heat insulating layer consisting of glass having high hardness does not deform much upon operation of a thermal head while a heat insulating layer consisting of a resin largely deforms because a resin is soft. That is, when a concentrated load is locally applied on a thermal head, deformation of the surface protective layer is smaller than that of the resin layer. Therefore, the protective layer cannot follow the deformation of the resin layer and cracks.
Various materials, therefore, have been examined as the surface protective layer. However, Ta.sub.2 O.sub.3 or SiO.sub.2, for example, is unsatisfactory in hardness, and Si.sub.3 N.sub.4, SiC, and Al.sub.2 O.sub.3 is unsatisfactory in toughness, and materials of both these types produce cracks. That is, none of the material has sufficient properties to be used in the thermal head using a resin for the heat insulating layer. Of these various materials, an example of a material not producing a crack is a SIALON film described in Japanese Patent Disclosure (KOKAI) Nos. 60-4077 and 62-3968. This SIALON film consists of Si, Al, O, and N as main components and has high hardness and toughness. A sputtering rate, however, upon formation of the SIALON film is low even in an Ar gas atmosphere. In addition, in this atmosphere, metal Al tends to precipitate to degrade an insulation property. Although this precipitation can be suppressed by adding 5% to 10% of O.sub.2 or N.sub.2 to the Ar gas, the sputtering rate is further decreased by this addition.